Coarse woody debris ameliorates chemical and biotic soil parameters of acidified broad-leaved forests

Base cation depletion due to soil acidification is a major environmental problem which can disrupt proper ecosystem functioning. We investigated nine mesic broad-leaved forests growing on Devonian sediments of the Rhenish Shield (Germany) and investigated the effect of coarse woody debris (CWD). The forests grow on soils originating from different parent material (graywacke, slates, dolostone) with upper soil pH-values ranging from 3.0 to 7.0. We analyzed soil chemistry, soil microbial parameters and the density of litter-dwelling detritivores (snails, slugs, woodlice and millipedes) to assess the influence of both acidification and CWD. The forests were classified according to their (Ca + Mg + K)/Al molar ratios into groups with ratios of either <1, 1–2 or ≫2. In general, microbial biomass and activity, as well as densities and species richness of snails and woodlice, decreased with decreasing (Ca + Mg + K)/Al molar ratios. Millipedes and slugs were not impaired by decreasing (Ca + Mg + K)/Al molar ratios. However, we found effects on species assemblages. Forward selection in canonical correspondence analyses revealed that species assemblages of millipedes and slugs were related to base cations and microbial parameters. Plots close to CWD exhibited improved soil quality including higher pH-values, greater Ca2+ and Mg2+ availability and an improved (Ca + Mg + K)/Al molar ratio. The presence of CWD also enhanced soil microbial biomass, the density of detritivores and species richness of snails, slugs and millipedes. Consequently, coarse woody debris influences nutrient fluxes and decreases the acidification-related risk of tree damage.